A typical forward-type DC-DC converter includes a switch circuit that alternately switches on and off an electric current flowing through the primary side of a transformer in a predetermined cycle. In response to the repeatedly alternating on-state and off-state of the primary-side current, an electric current appearing on the secondary side alternates between an on-state and an off-state. The secondary-side current that alternates between the on-state and the off-state is rectified by a rectifying circuit, and is then smoothed by a capacitor and an inductor, thereby generating an output having an approximately constant current and voltage.
In the DC-DC converter having the configuration described above, an electric current flowing through the transformer is of a rectangular wave form, which exhibits a sudden change at the edges thereof so as to include high frequency components. The higher the operating frequency, the greater the loss in the core of the transformer is.
It is thus not preferable to use an electric current having a rectangular wave shape.
A DC-DC converter disclosed in None-Patent Document 1 utilizes an LC resonance circuit to generate an electric current having a sinusoidal wave form whose frequency is equal to the resonance frequency. An electric current having a wave form synthesized from this sinusoidal-wave-form current is then supplied to the transformer. The fact that a sinusoidal wave has only one frequency component without having higher frequency components ensures that loss in the transformer is small, thereby allowing an efficient DC-DC converter to be provided.
However, the DC-DC converter disclosed in None-Patent Document 1 does not use PWM (pulse-width modulation) control to adjust the output voltage thereof, but changes the switching frequency on the primary side of the transformer to adjust the output voltage thereof. Such a configuration involves the use of a VCO (voltage-controlled oscillator) or the like, which results in a cost increase. Further, a small load connected to the output means that the switching frequency is to be increased. In such a case, the wave form input into the transformer ends up having a shape distorted from a sinusoidal wave form, which results in an increase in the loss in the transformer.
A DC-DC converter disclosed in Patent Document 1 utilizes an LC resonance circuit to generate an electric current having a sinusoidal wave form whose frequency is equal to the resonance frequency, thereby reducing switching loss occurring in transistors. In such a DC-DC converter, the sinusoidal wave form of electric current input into the transformer cannot be maintained under certain conditions of switching timing, which may cause an increase in the loss in the transformer.    [Patent Document 1] Japanese Laid-open Patent Publication No. 03-060367    [None-Patent Document 1] “Green Electronics No. 1, Designing of Highly Efficient, Low-Noise Power Supply Circuit”, p. 55, CQ Publishing Co., Ltd., Apr. 1, 2010